When Loiter mode motor would loss speed

Hello
I have had problems flying my drone on Loiter mode. I tried Alt Hold and STA on two situations, the motor speed is normal, but when using LOITER mode, one of the motors will always drop back to the idle speed.

I would like to ask if you have encountered such a problem and how to solve it.

Perhaps best asking this question directly in the arducopter forum:

But you will need to supply the .bin file and advise which version of arducopter you are currently using.
Right now you should be at least on version 1.4.4 but there has been a small update release 1.4.5.
If you you are on any earlier version there is not much point in trying to solve a perhaps old problem.
Also have you done all the calibrations and advanced tuning ? …a lack or incorrect way of doing those can often cause such problems.

OK, Thanks